One way to look at the stock market's incredible nine-year comeback is to consider an investing thought experiment: How much would you have if you bought $100 of an S&P 500-tracking index fund every month starting in March 2009?

As of Wednesday's close, an investor buying $100 of an S&P 500 fund at the closing price on the first trading day of each month since March 2, 2009, would have a portfolio worth $19,333 as of Wednesday's close, factoring in the price appreciation of the past nine years:

The above portfolio includes the value of the $100 in principal added each month, which comes to a total of $10,900 as of this month. Subtracting out that monthly investment and looking purely at the price return gives another illustration of how the stock market has done in the past nine years: